Refractive eye surgery to correct visual deficiencies and maladies includes, for example, lamellar corneal surgery, keratomileusis, epikeratophakia, cataract surgery, penetrating keratoplasty, conductive keratoplasty, corneal transplantation radial keratotomy, laser refractive keratectomy, and the like. Such therapeutic intervention involves incisions, punctures, sutures, etc. However, while ophthalmic microsurgery is often successfully performed, the results obtained can be subject to variation. In conductive keratoplasty, for example, a number of coagulation punctures are made into the cornea in order to change its curvature and correct refractive power. But, if the punctures are incorrectly placed, optimal vision will not result; and in some instances permanent distortions can result in corneal astigmatism, causing the vision of the patient to be worse than planned.
A keratometer is a standard instrument used for determining the curvature shape of the corneal surface of the eye. In use, the patient looks into the keratometer device, which then must be accurately positioned with respect to the patient's eye being examined along the line of view of the patient's eye (the optical axis between the pupil of the eye and the device). In some instances general centering of the device on the eye is sufficient, but in other cases exact centering of the device on the pupil or on the optical axis of the eye is critical to the success of the surgery.
The use of a circular illuminated ring on the keratometer causes a circular or elliptical reflection of the circular ring of light to appear on the patient's cornea. The reflection of the light by the tear film on the anterior surface of the cornea allows the ophthalmologist to more accurately view the surface contour of the eye, and target the center of the patient's eye during evaluation and refractive surgery. The reflection of the ring of light allows the ophthalmologist to measure the curvature of the cornea. Deviations of the cornea from sphericity cause bumps or indentations indicative of irregularities in the cornea to be present in the reflection. In an astigmatic eye the reflection of the circular ring of light appears elliptical to the ophthalmologist, and the degree of ellipticity is directly proportional to the amount of astigmatism present in the eye. The direction of the axis of the reflected ellipse corresponds to the direction of the astigmatism. Thus, the reflection provides information to the ophthalmologist regarding where incisions should be made and how much change is needed during the surgical intervention to correct the astigmatism in the patient's eye.
The keratometer's measurements relate to data actually measured at points peripheral to the apex of the patient's cornea, and as a result, the effectiveness and accuracy of the keratometer are directly dependent upon the accuracy of the positioning of the apparatus relative to the cornea. An accurate view of the eye, particularly for surgery near the pupil of the eye, depends directly upon the centering of the device, since misalignment of the equipment can result in skewed measurements and inaccuracies.
In general, the microsurgery is performed under a binocular microscope, which combines the two images, one produced by each ocular to each of the surgeon's eyes, into one image with depth perception. Parallax is the difference between two images. When a single image is viewed with both of an ophthalmologist's two eyes, each eye views the image from a slightly different angle because of the spacing between the two eyes. Each eye, therefore, has its own line of sight to the object being viewed, and when combined, a parallax occurs. By comparison, in a monocular view, i.e., using only one eye to view a two-dimensional image, no parallax occurs. However, the eye is 3-dimensional, not 2-dimensional. Consequently, the cornea at the center of the eye covering the pupil is not flat, but is raised 3-5 mm over the pupil, creating its own parallax when viewed binocularly. This error can be eliminated when the center of the pupil and the apex of the cornea are in alignment within the monocular line of sight of the ophthalmologist.
Improvements in the accuracy of the corneal visualization by ophthalmologists and microsurgeons will improve their ability to perform refractive surgery. Accordingly, a need continues to exist for a simple and precise method for focusing an operative keratometer, particularly in treating astigmatism, to permit the ophthalmologist to 1) qualitatively assess both the amount and direction of the astigmatism, 2) more accurately place incisions, punctures, sutures or other types of surgical intervention by providing a movable fixation point that provides direction for the patient to focus the subject eye, thereby placing the eye at a more convenient position for the ophthalmologist during surgery, and also to evaluate the effect of each incision, puncture, suture, etc. (postoperative compared with preoperative, or during surgery), and 3) to provide monocular centration of the optical axis and/or the pupil as needed, particularly for surgery near the pupil of the eye.